1. Field of the Invention
The present invention relates to a thin film shaped integrated circuit comprising a plurality of thin film insulated gate semiconductor devices such as thin film transistors (TFTs) formed on an insulating surface, and to a method of fabricating the integrated circuit. Semiconductor devices according to the invention can be used in active matrix circuits of electro-optical devices such as liquid crystal displays, in driver circuits for image sensors, in SOI integrated circuits, and in conventional semiconductor integrated circuits (e.g., microprocessors, microcontrollers, microcomputers, semiconductor memories, and so on). More particularly, the invention relates to a monolithic thin film integrated circuit comprising an active matrix circuit and drivers for driving the circuit are formed on the same substrate or memories and a CPU are formed on the same substrate and to a method of fabricating the monolithic thin film integrated circuit.
2. Description of the Related Art
In recent years, researches on a method of fabricating an insulated gate semiconductor device (MISFET) either on an insulating substrate or an insulating surface has been conducted. The insulating surface is insulated from the semiconductor substrate by a thick insulating film. A semiconductor device in which a semiconductor layer (active layer) has the form of a thin film is called a thin film transistor (TFT). It is difficult to obtain good crystallinity corresponding to the crystallinity of a single crystal semiconductor. Therefore, a non-single crystal semiconductor is commonly used.
Such a non-single crystal semiconductor is inferior in characteristics to a single crystal semiconductor. Especially, when a reverse voltage (a negative voltage in the case of an N-channel TFT and a positive voltage in the case of a P-channel TFT) is applied to the gate electrode, the leakage current between the source and drain increases and the mobility of the TFT reduces. Therefore, it is necessary to form an intrinsic or weak P- or N-type high resistance region between the source/drain regions and the gate electrode.
For example, where high resistance regions are formed, at least the side surfaces of a gate insulating film are oxidized by anodization or other method, and doping is done by a self-alignment utilizing the oxide or its trace. In this way, high resistance regions having a uniform width can be obtained.
However, such a high resistance region acts also as a resistor inserted in series between the source and drain. Therefore, where TFTs having different characteristics are formed on the same insulating surface, problems produce. Where high speed operation is needed, these regions are not necessary. For a monolithic circuit comprising the same substrate on which an active matrix circuit and drivers for driving the active matrix circuit are formed, the active matrix circuit has preferably low leakage current. Consequently, TFTs having wide high resistance regions are desired. On the other hand, the drivers are required to operate at high speeds. In consequence, TFTs having narrow high resistance regions are desired. However, high resistance regions of TFTs formed on the same substrate by the same process have a uniform width and so it is difficult to modify the widths of the high resistance regions according to the need. Furthermore, it is difficult to fabricate monolithic active matrix circuits and monolithic integrated circuits.